How Many People Should the Earth Support?

Ross McCluney

Excerpted with the permission of the author from Humanity's Environmental Future: Making Sense in a Troubled World by William Ross McCluney, SunPine Press, Cape Canaveral, Florida, 2004. ISBN 0-9744461-0-6. Available at www.thefutureofhumannity.com.

When environmentalists say that the world is overpopulated, they mean that the environmental consequences of the excessively high human population are destroying the biosphere--the Earth's life-support system. This leads to the question of what these environmental consequences are, and the related question of how many people can the Earth really support. As we'll see in this article, the question cannot be answered without offering another--
"What kind of world do you want?"Finally, there's another, more fundamental, question--"What kind of worlds are possible?"

Using a more ecocentric definition, what would be the desirable number of humans for the planet, rather than just how many we might be able to cram into "the pasture" on a "sustainable" basis.

I've been pondering these questions for some time. Joel Cohen wrote a book on the subject(1). In an astounding Appendix 3, Cohen tracks the estimated answers to the primary question from the earliest one listed (1679) to 1994. Estimates range from the very small (0.5 billion by Ehrlich) to the very large (a billion billion, obtained
by assuming that heat removal is the only limitation, resulting in a population
of around 500 people per square meter and an outer skin temperature of
the structures containing those people of 2000C). Most of the variation
over the years was due to the limited scientific knowledge available to
earlier prognosticators, and to very widely varying assumptions regarding
what physical or biological factor forces the limitation.

Assumptions Regarding Future Populations

Assumptions are necessary whenever we predict the future. Many believe that we don't have to look into the future to see that the problem has already gotten out of hand. Only observation is needed to see this. For future projections, however, one needs to make the best educated guess possible concerning what the future consequences are for projected future changes in policy, behavior, and/or environmental effects.

Ecologist and Agronomist David Pimentel of Cornell wrote a short article on the subject.(2) He made a fundamental assumption that all the people in the world had the current average standard of living in the United States. His estimate called for a substantial reduction in the current human population, indicating that the Earth's carrying capacity is inadequate to support the current world population at our present high standard of living. His numerical estimate came out to be from 1 to 2 billion people.

Since current world population is around 6 billion, the obvious question is how are we currently supporting a larger population than Pimentel's estimate? The answer is that most of the current six billion people have a much lower standard of living than the one assumed by Pimentel in his projection. Also, temporary sources of energy, separate from our daily budget of energy from the sun, are supporting a population that cannot be supported once these (fossil fuel) energy sources are depleted, as is projected to happen before the end of the next century. (Any estimate of long term maximum supportable human numbers must assume the absence
of substantial quantities of fossil fuel resources.)

Population Limits

To begin this discussion it must be pointed out that humans already take some 40% of primary photosynthetic production on Earth. This is an astounding figure, and illustrates how far humans have already gone in taking control of Spaceship Earth and altering the most fundamental life processes taking place on it.

A substantial increase in human population would therefore not be possible, except (1) at the expense of other life-forms with which humans compete but on which humans also depend, or (2) by finding a way to reduce this human impact on other life forms while human population continues to grow. The latter of these alternatives can only be considered wishful thinking,
a radically non-conservative philosophy repeating what a few misguided economists have said: the human mind can always invent some technological solution to future problems; therefore we should not worry about them so much. This refrain was stated explicity by Ted Koppel at the beginning of a recent ABC TV program on genetic engineering.

The first of these alternatives was put another way by Alan Thornhill, an ecologist at Rice University, when he said(3) that we are systematically converting non-human biomass into human biomass. The obvious conclusion is that at some point there will be insufficient non-human biomass to sustain human life. Of course many other life-support systems will break down, possibly irreversibly, before we reach this point.
In certain regions on the Earth, this turning point has already been reached and people are starving for lack of food within their own local growing regions.

Of course there are ultimate physical limits to how many people the Earth can support. Al Bartlett puts it this way: "World population in 1975 was estimated to be four billion and it was growing at the rate of 1.9% per year.... [If world population growth continues at this rate, it] would grow to a density of one person per square meter on the dry land surface of the earth (excluding Antarctica) in 550 years [, a population of 139 thousand billion people.].... Since it is obvious that people could never live at a density of one person per square meter over the land area of the Earth, it is obvious that the Earth will experience zero population growth (ZPG) at some point." Though this calculation is a bit silly, it leads to the question of what the long-term ZPG point will or should be. The question is not so simple to answer. This was made clear by Joel Cohen's book(1). Finding an exact value for the upper limit of human numbers on Earth is a very slippery slope. One thing is clear from the literature, though. The number depends very much on two variables, one easy to define, the other illusory.

Two Critical Variables

The first variable is the degree of affluence of the people "supported" by the Earth. We Americans know a lot about affluence, and our environmental organizations spend a great deal of time trying to reduce the environmental impacts of the consequences of our per capita affluence, through better, less polluting technology (and more efficient energy consumption), as well as through some lifestyle changes, such as recycling and improved business practices. It is important to note, as my colleague Paul Jindra* has pointed out to me, that efficiency is just a multiplier. No matter how "efficient" our energy-consuming processes are, the Earth still gets consumed. "Energy efficiency is a false god," he says.

The second variable is more difficult to pin down. It has to do with the future success of technology and new sources of energy in supporting an expanding population with expanding affluence. Those less concerned about population limits tend to believe that we can "technologize" our way out of any problem, no matter how large the scale. This is another way of saying, "The scientists will save us." So this second variable has to do with how benign the technologizing process can be over the next several decades, in support of the increasing numbers of people with increased affluence. Opponents of this salvation-through-science-and-technology" approach point out that no human technology can replace a species that is lost through so-called "human advancement."

Added to this second variable, and to some extent part of it, is the question of human adaptability. Can we be relatively happy and content as a species with fewer other species in the "natural" world around and with declines in green spaces? Perhaps the vision of those unconcerned about unlimited growth is that parks can be set aside to insure access to areas of at least seemingly unspoiled beauty, where at least the few remaining humans so inclined can get out and have a relatively traditional outdoor camping experience. Or perhaps it is the vision of some inventive Japanese who created an indoor ski slope not far from Tokyo, so skiers don't have to travel great distances to go snow skiing, and so they can do it year round, indoors.

Setting this sub-issue aside for the moment, assuming that we are so adaptable that we don't have to worry too much about retaining large scenic vistas and big sky wilderness areas for human enjoyment, we are still left with that difficult remaining variable concerning how far we can push the limits to growth with technological means alone. People apparently can live at very high population densities. Hong Kong and Tokyo come to mind. Of course such high densities require fairly large areas in addition to those cities--for the growing of food, the disposal of wastes, and the acquisition of other resources. The impacts of high Japanese population densities are felt far from Japan's shores. We are "answering" this question of how far technology and human population density can go with an experiment. We are now testing the world to see how far the premise can be pushed before consequences start happening that are generally considered undesirable. (Of course, with better public education, this point could be reached sooner rather than later, but this is too broad a subject to be considered here.)

Food Limits

Bruce Sundquist studied the food-growing limits to human population numbers and has this to say(5):

Probably what people are really contemplating is the maximum population the Earth can sustain indefinitely in some sort of steady-state. Or perhaps they really mean how many people can the Earth support until fossil fuels are depleted. I have been working on the steady-state problem for over a decade now, but considering mainly topsoil as the limiting resource. I now have annotated reviews of the global literature on topsoil loss, forest land degradation, grazing land degradation, irrigated land degradation, and fishery degradation. It has become clear that there is no need to worry about energy-- agricultural top soil and those dependent on it will vanish long before the last barrel of liquefied coal is gone.

The lifetime of past civilizations correlates well with their topsoil resources. Civilizations that had a river system that constantly replenished topsoil resources always lasted far longer than civilizations that did not. I argue that topsoil is still the limiting factor for modern-day civilizations as well. Below I run through a very rough outline of my arguments to give people an idea of key facts and figures. Ignored are such niceties as human rights, biodiversity, aesthetics etc. Man is considered as purely a mindless animal consuming food. This is the way to arrive at the most optimistic conclusions possible.

Irrigated land provides roughly 40% of the world's soil-based food supply. In the opinion of experts in the field, the ultimate fate of the world's irrigation systems will be much the same as the fate of irrigation systems of old--barren salt flats. This is apparently because few systems are underlain by drainage tiles for draining water away. Irrigation-system growth was one of the three main reasons why growth of global food supplies kept up with global population growth over the past 4 decades. Presently however, creation of new systems is roughly balanced by (and probably less than) the rate of irrigation-system abandonment due to salination and reallocation of water supplies to urban uses. The rate of abandonment is sure to increase dramatically in decades ahead because it takes some time for the effects of salt buildup to appear, and most irrigation systems are only decades old.

If one takes the most optimistic data on grassland photosynthesis and data on how much meat is produced per ton of grass and ton of grain, it becomes clear that the world's grasslands are overgrazed by a factor of about 2. This is easy to see from river-sediment data. Rivers draining the world's arid grasslands are "turbid" and remove several times as much sediment per acre per year as average sediment loss rates from average developed land. Most grazing-land sediment is sub-soil from erosion by gullies and stream banks, so actual topsoil losses may not be much greater than on croplands, but the basic erosion mechanisms (gullies, stream-bank erosion) are usually indicative of topsoil erosion on a massive scale--i.e. over grazing.

If one adds up the rates of topsoil deposition in oceans, river bottoms, dam backwaters and alluvial plains; then adds topsoil losses due to wind erosion, salination of irrigation systems, urbanization, and several other minor effects; then subtracts off topsoil losses from forest lands and urban lands, one gets a net topsoil loss rate from agricultural land of roughly 100 billion metric tonnes per year (100 Gt/year)--at least 5 times the rate of natural topsoil-creation on agricultural lands.

The global inventory of cropland topsoil is about 6500 Gt. Grazing land topsoil inventories are perhaps three time that (though an acre of grazing land is only about 1/6 as dollar-productive as an acre of cropland). The global inventory of top soil on potential (not yet used) croplands is several times 6500 Gt, but considering only potential croplands that can be cropped sustainably, the potential inventory is only perhaps 10 percent that of existing croplands--barely enough to replenish croplands abandoned due to degradation and urbanization for a few decades. This perhaps explains why the global cropland inventory has been constant since the early 1980s.

I am still working on how to apportion global agricultural topsoil losses between croplands and grazing lands, but I suspect the gross rates are not that much different. This would suggest a cropland topsoil loss rate of 50 Gt per year from a maximum inventory (actual plus potential) of at most 7000 Gt, suggesting a lifetime of human civilization of 7000/50 or 140 years. But now consider that once topsoil depths drop below the depth of the root zone (about 6 inches) cropland erosion becomes nearly irreversible and increases rapid. Current optimistic average depths of cropland topsoil are not over 11 inches, and some data say several inches less. So civilization has only about half of its topsoil to spend down before things get really bad. This gives a lifetime for human civilization of about 70 years--barely one human lifetime, and just an eye-blink in terms of human history.

How is this analysis translated into the number of people that the Earth can support sustainably? Assume that net agricultural topsoil loss rates are directly proportional to human population--an assumption that correlates well with global variations in topsoil loss. In order to reduce gross agricultural topsoil loss to the natural rate of agricultural topsoil creation, the Earth's population would need to fall to about a fifth of its present value--perhaps 1.2 billion. Escalation of irrigated land degradation due to salination could drop this figure to well under one billion.

Neglected here is cropland productivity growth due to genetic advances and increased use of fertilizer--the other two effects that largely supported population growth during the past 4 decades. However both of these effects are now close to saturation, so
one should not expect really substantive increases in maximum population values from either of these effects. Increased use of pesticides to attempt to reduce crop losses from the present 10-20% of total production has never shown the ability to cut crop losses to pests, probably because increasing use of monocultures and shrinkage of the global plant gene-pool have worked to counteract whatever benefits pesticides might otherwise be expected to offer.

A far more likely steady-state scenario than human population falling to 1.2 billion is that cropland topsoil is largely destroyed, and the Earth becomes a waste land with populations held constant by war, disease, hunger, suicide and genocide. The productivity of sub-soil is not well known, though it is probably not over 10 percent of the productivity of topsoil. Hence the maximum population under the far more realistic steady-state scenario is probably under 10 percent of the maximum population that a not-erosion-limited, topsoil-based civilization can sustain--possibly 0.6 billion.

Fisheries have been neglected in all this. The problem with fisheries is that Man keeps fishing further and further down the oceanic- and fresh-water food chains, and the lower we go the more dispersed fisheries become. At the dispersion value of the open ocean where about 75 percent of oceanic life-creation occurs, fuel costs for fishing boats per ton of fish harvested increases by about a factor of 100 from present-day values. And present-day fishing-boat fuel costs are already a significant portion of the price people now pay for fish. Aquaculture
imposes yet another demand on world grain supplies. So although it may provide a positive contribution to protein sources, its contribution to caloric supply is probably negative. And consider that aquaculture usually entails destruction of coastal wetlands, estuaries and mangrove swamps, all of which provide vital breeding grounds for 80-90 percent of ocean fish, and the frequently-diseased fish in ocean aquaculture pens often escape and devastate populations of their wild cousins. So it is not clear that aquaculture provides a net benefit of any kind.

And let us not forget hydroponics that some say permit a global population of 50 billion or so. Hydroponics is useful for producing the more expensive foods (fruits and vegetables) for wealthy First-World people. But the idea of using hydroponics to produce
complete diets of average First World people, to say nothing of Third World people, strains credulity. Imagine how many fluorescent light bulbs would be needed to replace the sunlight over all the croplands of the world.

This contribution illustrates how difficult it is to pin down an exact number for the carrying capacity of the Earth for humans, and illustrates how complex the human life-support system is and shows the kinds of assumptions that must be made to produce an estimated maximum
population figure. The energy component of population estimates cannot be ignored. In the United States, for every calorie of food energy consumed by humans, on the average it takes 10 calories of fossil fuel energy to grow the food transport it to the table and prepare and package it. In
the United States the ratio of fossil fuel to solar energy in the food you consume is 10 to 1! We don't eat solar energy--we eat fossil fuel. As that is depleted, it doesn't take a genius to anticipate the consequences for the average U.S. diet.

Using his methods Sundquist comes up with sustainable human population estimates ranging from 0.6 to 1.2 billion, well below the current 6 billion figure, based on topsoil loss estimates and other factors. The low figure of 0.5 billion mentioned by Cohen came
from this statement by Paul Ehrlich in 1971: "There are 3.6 billion human beings on the face of the Earth. According to our best estimates, there are somewhere between three and seven times more people than this planet can possibly maintain over a long period of time." [Paul R. Ehrlich, "The Population Crisis--Where We Stand," Population, Environment, and People, Noel Hinrichs, ed., McGraw-Hill, NY, 1971, pp. 8-16.] Sheila Newman(6) suggests the concept of an ecological footprint can be helpful in making another estimate of sustainable human populations:

Ecological Footprint is a term coined by
Mathias Wackernagel with The Task Force on Planning Healthy & Sustainable Communities at The University of British Columbia. [See additional information.] It symbolizes the amount of productive land required to sustain human life according to different economies. Using the "Concept of Appropriated Carrying Capacity for Measuring Sustainability", the method uses a formula for the calculation of land areas required for human activities, choosing ethanol as the renewable substitute for fossil fuel and assuming an ethanol productivity per hectare of an optimistic 80 gigajoules per hectare per year [Gj/(ha-yr)] on biologically productive land. This kind of measurement makes it possible to quantify
and compare energy use across different species and different human societies.

Wakernagel data on Holland gives its population in 1997 as 15,697,000, with an ecological footprint of 5.3 hectares per capita, in a country with only 1.7 hectares per capita available. This means that the Dutch population is using 83,194,100 hectares of bologically
productive land altogether in a country that only has 26,684,900 hectares of such land available. Obviously Holland is importing a great deal of energy and materials produced by biologically productive land located elsewhere.

Although these methods seem to be the best around for estimating regional population impacts for now, the operational definitions are necessarily still very crude. Of many possible reasons for this, the main one is the approximate nature of the information available for each country. As well as lacking detailed information in many areas, the world lacks standardized definitions in terms of quantity and type of oil consumption, variations in quality of biologically productive land, variations in quality of biologically productive sea, and the amount of land necessary for preservation of wilderness biodiversity.

Members of the Wackernagel team have estimated that there are approximately 1.7 hectares of biologically productive space (with world average productivity) available per world
citizen. On the basis of their formula they have calculated that if global population continues to grow as expected, in 2030 a predicted world population of 10 billion people will each have an average of only 0.9 hectares of productive land available. (This does not take into account the probability of more soil degradation.) This is an example showing how population size
increases demand on "nature's productivity."

Keeping these issues in mind, let's look at another one of the assumptions needed. Sundquist ignored human rights, biodiversity, and aesthetic appreciations of natural beauty, considering humans only as mindless animals consuming food. Let's put these variables back into the equation and ask how many the Earth can support at various levels of human affluence.

Pimentel has estimated that the Earth can support from 1 to 2 billion people with an American standard of living, good health, nutrition, prosperity, personal dignity, and freedom. Using his two billion figure as the benchmark, several additional estimates of population limits can be offered.

Accurate numbers for each of these possible scenarios, or assumptions concerning living conditions, are very difficult to come by. I have done no original research in exploring this topic, and only offer the following as food for thought. If you are interested in having more detailed knowledge, you might like to read Joel Cohen's very well researched and scholarly book on the subject.

Few rational people would claim that the Earth could support a population density of one person per square meter of land area, a population of some 140,000 billion people. Where would you grow the food? Short of that absurd limit, let us look at somewhat more reasonable estimates. These are my "rabbit out of the hat" estimates only, intended to provoke thought and illustrate the difficulties of coming up with good future predictions. There is at least one conclusion that can be drawn from these guesses, however. Trying to answer the question of how many people the Earth can support only brings another question, "What kind of world do you want?"

Maximum Global Population Guesses

Each of these assumes that the current depletion of fossil fuel reserves has continued to completion. No fossil fuels are left, except possibly for a small stock, priced high, and used for limited durable uses such as new plastic production and for some pharmaceuticals.

1. Everyone at the current U.S. standard of living and with all the health, nutrition, personal dignity and freedom that most Americans currently enjoy [Pimentel, 1999].

2 billion

2. Everyone at the same affluence level as in 1, but with few restrictions on commerce, pollution, land use, personal behavior (within current law), etc. Basically a libertarian, laissez faire economy, with only limited environmental restrictions. This points out that there is a population price to pay for the current American way of commerce.

0.5 billion

3. Everyone at the same affluence as indicated in 1, but with many and onerous restrictions on freedoms relative to behaviors leading to environmental degradation. In order to accommodate populationlevels greater than 2 billion, restrictions such as the following would
have to be instituted: Massive recycling. Driving restrictions (gasolene rationing, fuel rationing even to mass transit systems). Restrictions on the transport of food (food transported no more than 100 miles for example to its point of retail sales). Prohibitions against cutting of trees on one's property. Limitations on the burning of fossil fuels in order to save these complex molecules for more valuable or durable uses, such as in the manufacture of plastics and pharmaceuticals. Limitations on the areas of open spaces that can be converted to renewable energy power plants, such as solar thermal, solar photovoltaic, and wind energy systems. This latter results from the need to preserve natural areas for atmospheric oxygen generation and food growing. Of course many rooftops can accept solar energy systems and this scenario basically assumes a nearly complete saturation of coverage of roof tops and covers over parking lots for solar energy production.

4 billion

4. Only people in the U.S. and Europe at current level of affluence. Everyone else at the current prosperity level of Mexico.

6 billion

5. Everyone in the world at Mexico's current prosperity level.

20 billion

6. Everyone in the world at the current "prosperity" level of Northwest Africa.

40 billion

Conclusions

Of course the point of this exercise is to point out that if we wish to grow the world population to the UN projection of about 12 billion near the middle of the next century, such growth will have to come at the expense of many things, not the least of which is compassion for people less fortunate than we in the U.S.

It also shows somewhat clearly what I have been saying for over 30 years, that increasing population density is inextricably linked to loss of freedom and losses of choice. In the worst of the above scenarios, we can forget the Bill of Rights. This was pointed out recently
by M. Boyd Wilcox in his article, "March 27, 1999: On the anniversary of the Rockefeller Report, Overpopulation Dilutes Democracy," which appeared in the March/April issue of Population Press(8). A quote from that article: "One need not pander to Malthusian or apocalyptic thinking to ask in all seriousness whether the biosphere can survive another century like this one. Arguably, it is not biological survival of the human species that is in danger so much as it is the moral or spiritual survival of what it means to be human and to be part of a complex living community. We cannot count the ways in which human identity, imagination, and esthetic appreciation depend on the richly textured landscape of nonhuman nature. What but unbridled hubris could let us think that what we consider human nature will survive if we despoil all of nonhuman nature?"

This exercise also leads to Paul Jindra's conclusion that "Ultimately, there is no such thing as the rule of law, only the rule of numbers." By this he means that as the population grows, freedoms of behavior and choice, and niceties such as human rights, take a back seat to more primordial struggles of humans to survive, by whatever means possible. In Jonathon Porritt's penetrating book Save the Earth(4),
it is pointed out that "In the majority of large Third World cities, over 70 percent of all new housing is constructed 'illegally' on unofficial settlement areas. In some cities the figure can be as high as 95 percent." Social niceties such as "legality" are washed away by the overwhelming
rule of numbers.

So we come to this inescapable conclusion when we try to answer the question of how many people the Earth can support--the answer can only be a counter question: "What kind of world do you want?" If you want a world where Americans can continue living pretty much as they are, then global human population is likely to be substantially less than it is now. If you wish to keep the current American standard of living where it is, while allowing the rest of the world to grow substantially in numbers, the consequence is to doom that "other world" to perpetual
misery and lost expectations, while doggedly holding on to the American way of life as desperately as possible.

This is not a world I want for my children and grandchildren, nor is it one I think anyone else would care to see. The only response can be to drop all these time and energy-wasting lost-cause efforts to keep both freedoms of fertility, immigration, and affluence while continuing to grow world human population. Instead we should work to reduce the number of humans living on Earth. This can be done peacefully by conscious action, or it can be done coercively, or involuntarily--as is happening currently in Zimbabwe, with its 30% level of AIDS infection.

To a great extent, the arguments advanced above are like having a multidisciplinary group of scholars argue for days concerning how many angels can dance on the head of a pin. We share this world with other creatures, none of which is capable of talking to us directly. Dennis Philips asks: "What if they could?"(7):

If we "take a vote", after abandoning our Anthropocentrism (with an Earth "Senate" in which each species gets one vote, and an Earth "House" in which voting is proportional to Biomass), I'm skeptical as to whether the other members of the crew and passenger contingent of "Spaceship Gaia" would even agree with Oregon Optimal Population Society's "modest proposal" for some 10 million humans worldwide.

We need to start thinking less about how many people the Earth can support and focus more on how many it should support. Using a more ecocentric definition, what would be the desirable number of humans for the planet, rather than just how many we might be able to cram into "the pasture" on a "sustainable" basis. My view is that with a substantially reduced and stable world human population, of only ten million people or so, we could keep our current material affluence, not worry so much about technological fixes to our environmental problems, because the magnitude of those problems would be greatly reduced and, by and large, manageable by the huge biosphere. This would provide us with the highest freedom. People liking high density could live in well-designed cities. People at the other end of the spectrum could live like hermits in the new wildernesses that would return to Earth, in abundance, after a time. Wastes could be discarded freely, or transported some distance away from the people to avoid contact. Recycling could be minimized. What pollution and habitat losses humans would cause could easily be absorbed by the Earth without great or permanent impact (except for a need to protect already endangered species habitat until that habitat could spread sufficiently to make the species viable again).

The choice of which path to take into the Brave New World is ours, as a species, to make. We can proceed blindly growing, as we now are, and hope against hope that somehow diseases like AIDS and wars (to other people, in other parts of the world), and lowered fertility rates due to improved affluence will reduce the most rapidly growing populations without affecting us in our own seemingly distant part of the world. We have to acknowledge, however, that such a philosophy is not a compassionate one, and says something I'd not like to say about the kind of world we want for our fellow humans.

Due to transportation and communication technologies, this isolationist position probably cannot really insulate us from the terrible consequences of such a policy. Diseases spreading like wildfire over much of Africa, or anywhere else, are not likely to remain there. Global warming and inadvertent climate modification affect the whole Earth.

Alternatively, we can take conscious steps now, and become true to the special "gift" of intelligence that has come to us above all other species, using that intelligence to reduce our numbers, providing a better future for all. The choice is ours to make, but it will take a lot of education and persistence.

2. Pimentel, David, "How many people can the Earth support?", Population Press, March/April 1999 (Vol. 5, No. 3), the Pop!ulation Coalition. Available at www.popco.org.
Click on "Population Press" and "Newsletters & Publications" then look under the heading for the April 1999 issue, or access the article directly at
http://www.popco.org/press/pimentel.html.